Exp 6 Gas Temperature Process Control

EXP 6 - GAS TEMPERATURE (AT 922) PROCESS CONTROL

TABLE OF CONTENTS

Bil. Title Page

1 Objectives 2

2 Summary 3

3 Introduction and Theory 4

4 Results and Discussions 5

5 Conclusion 10

6 References 11

7 Appendices 12

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OBJECTIVES

There are several objectives in experiment gas temperature (at 922) process control. The

objectives of this experiment are:

1. To identify the important components of the air temperature control system and to

mark them in the P&I Diagram

2. To carry out the start-up procedures systematically.

3. To determine the values of the parameters for a first order plus dead time transfer

function model of a thermal process.

4. To control the Air-Heater process using PID controller

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SUMMARY

The objectives of this experiment is to identify the important components of the air

temperature control system and to mark them in the P&I Diagram, to carry out the start-up

procedures systematically, to determine the values of the parameters for a first order plus

dead time transfer function model of a thermal process and to control the Air-Heater process

using PID controller. Lag occurred when the process started. This is also known as dead time.

It uses a transistor to adjust the heat flow to the heater. The air flow rate is measured using a

rotameter. PID algorithm is used for direct field control. That is either both of its input (PV)

and output (MV) is directly connected to field or process equipment. Three different PID trial

values show different results based on the tuning. Experiment was also done on load

disturbance and set point change to observe the responses Temperature measuring device

used in these experiment are RTD and type K. Using PID controller as a process controller,

tuning a temperature controller involves setting the proportional, integral and derivatives to

get the best possible control for a particular process. The MV is first adjusted to 24.4% and

then it is increased to 50%, it is shown that there is disturbance occurred when the valve is

being closed, the dead time was 28.8 secs. The tangent for the increase is calculated using

Pythagoras theorem. The tangent calculated is the response rate of the process.

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INTRODUCTION

The objective of this experiment is to determine the gas temperature process control

type AT 922 uses to stimulate a gas or vapor phase temperature process. The temperature

process is a lag/dead time process with no noise. The dead time in time unit is:

Dead time = [Distance∈mm x 3600 s ]

Record chart speed , 500 mm/ Hr

It uses a transistor to adjust the heat flow to the heater. The air flow rate is measured

using a rotameter. A selective control technique is employed here that automatically select

only a less heat demanding output to manipulate only one final control element (the

transistor/heater). This system requires a high gain PID controller.

Temperature measuring device used in these experiment are RTD and type K. A

resistance temperature detector (RTD) is a temperature sensing probe of finely wound

platinum wire that displays a linear resistance increase for a corresponding temperature

increase. RTDs are built on the principle that most metals have a positive charge in electrical

resistance with a change in temperature. When quality control is importance, a RTD is

unequalled for accuracy and repeatability.

Using PID controller as a process controller, tuning a temperature controller involves

setting the proportional, integral and derivatives to get the best possible control for a

particular process. The PID function uses system feedback to continuously control a dynamic

process. The purpose of PID control is to keep process running as close as possible to a

desired set point.

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RESULT AND DISCUSSION

First order plus time delay (FOPTD) transfer function as follows:

G(s) =K

Ts+1e−Ѳs

The chart below shows how dead time value can be determined. Distance of point 1 to

point 2 is measured by using ruler and substituted to the formula to get value. Based on the

graph, it shows that as the heater is on the heated temperature is starting to increase slowly.

The MV is first adjusted to 24.4% and then it is increased to 50%, it is shown that there is

disturbance occurred when the valve is being closed. This experiment chart was run at

500mm/Hr.

Figure 1: dead time

Dead time = ( Dist ance∈mm x3600 sec)


= (4 mm x3600 sec)


= 28.8 secs

The dead time is the time after each event during which the system is not able to record

another event. After the increase, for 4 seconds, there is a dead time as shown in the chart.

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0.2 cm

1 cm


Dead time can arise in a control loop for a number of reasons such as the time it takes for

material to travel from one point to another. The travel time is dead time. Not only that,

sensors can take precious time to yield their measurement result. The dead time is calculated

using a ruler and the limitation of the calculation is that the y-axis on the graph is precise only

up to one second. This is a problem because the time constants are in the order of one second.

Hence in calculation it is not possible to achieve a higher precision because of the lack of

measuring instruments with higher precision.

Figure 2: Response rate

Next, response rate must be calculated too using the chart which is shown in figure 2. The

tangent for the increase is calculated using Pythagoras theorem. The tangent calculated is the

response rate of the process.

Figure 3: Response rate, RR = 1.02 cm

Using the response rate, the process gain, K and time constant of the process value,Ʈ

are obtained from the calculation using formula shown below.

For process gain, K can be calculated as follow:

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1.02 cm


K= max change∈air temperature%change∈themanipulated variable

K= 4250−24.4

K=1.64

The constant time,Ʈ can be calculated as follow:

Maximum change in the air temperature at the exit = 1980C – 1560C = 420C

Ʈ= RRmax change∈air temperature

Ʈ=10242

Ʈ=2.429

The parameter of the First Order with Delay model is:

G(s) =K

Ts+1e−Ѳ s

G(s) =1.64

2.429 s+1e−28.8 s

Next, the air heater is controlled using the PID controller with load disturbances and set point

changes which are set point SV= 80ºC and 90 ºC at the temperature PID controller TIC91 and

is still in Manual (M) mode and its MV = 0%. For load disturbances the MV value changes

from 0 to 30%.

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Figure 4: First trial values: PB (%) = 10%, TI (secs) = 100secs, TD (secs) = 25secs

For the first trial PID controller, the response for the air heated temperature is fast. It can be

said that when the load disturbance is being introduced the temperature is increases due to the

rapid opening of valve for the air flow rate. At set point, SV=80ᵒC, it reaches a steady state

after few cycles. While for SV=80ᵒC and load disturbance MV= 30%, it shows the overshoot

is smaller compared to the previous one. To reduce the offset, the temperature for set point

and the temperature after load disturbance introduced must be stable.

Figure 5: Second trial values: PB (%) = 20%, TI (secs) = 70secs, TD (secs) = 18secs

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For the second trial PID values, the response for the air heated temperature started to oscillate

mildly as the process gain is increased. As the MV is increased to 50%, the response become

stable due to the opening of valve, then the response became little oscillatory when MV is

increased to 90%. As in theory, the response is slower but the offset can be reduced much

faster.

Figure 6: Third trial values: PB (%) = 20%, TI (secs) = 150secs, TD (secs) = 37secs

For the third values, the hart is becoming more stable. That is why it has a very small

oscillation. When the set point is changed to 900C, the response shows some oscillation and it

is undershoot as the temperature is being increased. After the response is stable, the set point

is then increased to 1000C and it shows that response is overshoots due to the changes of set

point and the response is became stable again.

Error might happen in opening of valve and air inlet valve. This will produce error in the

process. Since for measuring gas temperature uses heater instrument kept in air temperature,

the temperature reading may fluctuate.

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CONCLUSION

Firstly, the experiment shows that lag occurred when the process started. This is also

known as dead time. Dead time can arise in a control loop for a number of reasons such as the

long distance for material to travel from one point to another. However, there are limitations

to calculate dead time, for example high precision equipment acquired. Next is the control of

process using PID controller. PID algorithm is used for direct field control. That is either both

of its input (PV) and output (MV) is directly connected to field or process equipment. It is

designed to cope with any electrical noise induced into its circuits by equipment in the plant

or factory. Three different PID trial values show different results based on the tuning.

Experiment was also done on load disturbance and set point change to observe the responses,

whether they are overshoot or undershoot according to changes made. Several errors occurred

during the experiment was done and the results are no exactly precise or accurate. Some

recommendations to improve the results are following the procedures accordingly and

monitoring the panel closely.

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REFERENCES

1. Altmann W. (2005). Practical Process Control of Engineers and Technicians.

Oxford: Elsevier.

2. Carlos A. Smith, Armando Corrpion (2006). Principles and practice of Automatic

Control [Third Edition]

3. http://neogeo.kent.edu/munro/physical/Notes_Fall09/air%20temp.pdf(acsessed on 3

October 2011)

4. http://iseinc.com/what%20is%20pid.htm(acsessed on 3 October 2011)

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APPENDICES

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Exp 6 Gas Temperature Process Control

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